Patent GB-701783-A (GB)
Title: Improvements relating to television receiving apparatus
Inventor: KEEN ARTHUR WILLIAM
IPC: H04N5/213
Filed: 1950-03-24
Abstract: 701,783. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. March 14, 1951 [March 24, 1950], No. 7371/50. Class 40 (3). [Also in Group XL(c)] In television receiving apparatus, the frame deflections are initiated by locally generated control pulses which are synchronous with the line synchronizing pulses and are fed to the frame deflecting circuit via a gating device which is opened at appropriate times under the control of the frame synchronizing pulses. The arrangement eliminates the effects of interference in varving the itme at which frame synchronizing pulses become effective and permits accurate interlacing of the component pictures frames. In a double interlaced system, two series of control pulses are generated, the pulses in one series occurring at half line intervals after the pulses in the other series, and are fed to the gating device via further gates which are opened alternately when the frame deflecting circuit operates. In this way alternate operations of the frame deflecting circuit occur at the end of a line period and at the middle of a line period so producing correct interlacing of the picture frames. In Fig. 2 multivibrator 12, 13 is maintained in synchronism with the line synchronizing pulses by a control voltage applied at terminal 19 and generates two trains of square waves at line frequency, the waves in one train being 180 degrees out of phase and therefore displaced in time by half a line period. The trains of waves are differentiated by circuits 20, 21 and 25, 26, limited to negative excursions by diodes 22 and 27, and fed to the control grids of gate stages 33 and 36 respectively. Stages 33 and 36 are connected as cathode followers with a common cathode load 38 and feed a further gate stage 41, 45 controlling a frame frequency multivibrator 50, 51 in the frame deflecting circuit. Multivibrator 50, 51 may drive the sawtooth generator or be arranged itself to produce a sawtooth wave. The gate stages 33 and 36 are controlled by a trigger circuit 62, 63 so as to become transmissive alternately during successive frame periods, the multivibrator 62, 63 being reversed at each operation of the frame deflecting circuit by a connection extending through circuit 71-73. The mixed line and frame synchronizing pulses are fed via condenser 77 whence they are d c. restored by diode 76 and then integrated by network 79-82 to effect amplitude separation of the frame from the line pulses. The composite frame pulses are then amplified, squared and inverted bv stage 87, 88 and the output employed to render gate 41, 45 transmissive. Diode 83 prevents interference pulses from exceeding the amplitude of the frame pulses. When a frame pulse opens gate 41, 45, pulses at line frequency are fed to multivibrator 50, 51 either from gate 33 or 36 according to which of the pair is transmitting. Thus, assuming a frame deflection has been initiated by a pulse from gate 33, multivibrator 62, 63 is reversed closing gate 33, and opening gate 36. Upon the occurrence of the next frame pulse when gate 41, 45 opens again, multivibrator 50, 51 is triggered by a pulse from gate 36, and since the pulses derived from the gates 33 and 36 are displaced in time by half a line period, succeeding frame deflections will be initiated at appropriate times to provide correct interlacing. The adjustable tapping on resistor 18 in the circuit of multivibrator 12, 13 permits the timing of the pulse train and hence the degree of interlace to be adjusted. Differentiating circuits 20, 21 and 25, 26 may be omitted and the square waves employed as control pulses. Line deflections are controlled by the output from tube 12 which is fed via a network 24. Network 24 may serve, as described in Specification 701,785, to generate groups of three pulses at tappings A, B and C, the centre pulse of each group being employed for driving the line scanning circuit, and the other two pulses serving to synchronize the line scanning with the received line pulses and to develop a frequency control voltage for application to multivibrator 12, 13, at terminal 19. In a modified arrangement, Fig. 3 (not shown), the two trains of pulses at line frequency are generated by a circuit comprising a sawtooth generator in the form of a blocking oscillator followed by a linearizing stage which causes two differently-biased diodes to become conductive at different times to provide the pulses. A further modification, Fig. 4 (not shown) employs a circuit with a diode to the cathode and anode of which are applied sawtooth waves at line and half frame frequency respectively. Periodically the value of the sawtooth waves become equal to cause the diode to conduct and generate a pulse. Due to the frequencies of the two sawtooth waves the timing of the generated pulses varies slowly and in a progressive manner so as to change to the extent of half a line period during a picture frame. The invention may be applied to receiver in which more than two frames constitute each picture by modifying the arrangement of Fig. 3 by the use of an appropriate plurality of diodes to generate the required number of displaced pulse train. Fig. 4 may also be modified by the use of sawtooth waves at line frequencies and f where f is the same frequency and n the number of frames per picture. Specifications 471,731 and 655,790, [Group XL (c)], also are referred to.